1. Technical Field
This invention relates to ice detectors, and more particularly, to ice detectors for detecting the presence of supercooled large droplets that freeze and form ice on aircraft surfaces.
2. Background Art
Ice detectors are commonly used on a variety of aircraft to advise the pilot that he or she is flying in conditions that may result in the formation of ice on the aircraft. See, for example, U.S. Pat. Nos. 4,611,492; 4,553,137; and 3,341,835. If ice forms on wing surfaces or other control surfaces such as flaps or ailerons, it can degrade the aerodynamic performance of the aircraft, and in some cases, may make the aircraft difficult to control. In extreme instances of ice formation on the aircraft, the pilot may be unable to control the flight direction, and the aircraft can crash. Most ice detectors include electronics that provide a signal to the pilot of icing conditions, which allow the pilot to either activate the aircraft's ice protection systems or to change the aircraft's course and fly out of the icing conditions. Conventional ice protection systems are generally based on thermal, chemical or mechanical principals (or a combination thereof) and include electric heaters, pneumatic boots, or bypass air heaters; see, for example, U.S. Pat. Nos. 5,743,494; 5,657,951; and 5,562,265. Advanced ice detectors, commonly referred to as primary ice detectors, automatically activate the aircraft's anti-icing equipment when icing conditions are detected, without further action required by the pilot.
In general, ice will form on an aircraft surface if the aircraft is flying through water droplets whose temperature is below the freezing point of water, namely 0.degree. C. (32.degree. F.). Droplets below the freezing point are often referred to as "supercooled" droplets.
A typical precipitation-carrying cloud is comprised of water droplets having an average diameter of about twenty microns (0.00008 inches). An airplane flying through a cloud containing twenty micron diameter droplets, even if the droplets are supercooled, usually does not experience dangerously high levels of icing, because of the aerodynamic effect the aircraft surface has on the direction of the droplets as they flow over the surface of the aircraft. For example, the wing disturbs the flow of air over the wing surface such that the majority of water droplets in the twenty micron diameter range do not actually strike the surface, but rather flow past it. This is because, in general, the droplets have insufficient momentum to continue moving in a flow direction that would otherwise cause them to strike the wing surface; because of their small momentum (primarily a result of their small size) the disturbance of air flow in which they are entrained causes the droplets to follow the direction of the airflow and miss the wing. It has been observed, however, that some small droplets do in fact strike and freeze on the wing surface to form ice. This usually happens at the leading edge of the wing, and as indicated above, most aircraft are built with anti-icing or other ice protection devices to remove ice that forms at the leading edge of the wing. Ice has also been observed to form on other control surfaces of the aircraft and these, too, are often outfitted with means for removing ice that will form.
Several recent incidents involving actual crashes of commercial aircraft have shown that ice sometimes forms on wing surfaces aft, or downstream of, the ice protection systems on the wing and other control surfaces. Research has shown that such icing is caused by supercooled water droplets larger than the typical twenty micron diameter sized cloud droplets. See, for example, "NASA/FAA/NCAR Supercooled Large Droplet Icing Flight Research: Summary of Winter 96-97 Flight Operations" by Miller et al., ALAA 98-0577; and "Droplet Size Distribution and Ice Shapes" by Shah et al., AIAA 98-0487. It is believed that these large droplets are generally in the range of about fifty microns or more in diameter. Research has shown that icing caused by these so-called supercooled large droplets (sometimes referred to as "SLD icing") occurs because the SLD droplets have considerably more momentum (due to their large diameter and associated larger mass) than the typical twenty micron diameter cloud droplets; these SLD droplets travel in a flow path that is generally not disturbed or changed by the aircraft wing. As a result, these large droplets strike the leading edge of the wing as well as portions of the wing downstream of its leading edge. SLD ice that forms on the wing leading edge surface is generally not a problem because it is removed by the conventional ice protection systems referred to above; however there is usually no way to remove ice that forms on the aft sections of the wing and other control surfaces when the aircraft is airborne. This icing not only disrupts the airflow over the wing and other control surfaces, but also adds extra weight to the aircraft, thereby compromising the pilot's ability to properly and safely navigate the aircraft.
Conventional, prior art ice detectors are not able to advise the pilot whether ice forming on the detector is caused by the small (twenty micron) water droplets or whether it is caused by large supercooled (fifty micron or larger) droplets. Often, the air through which the pilot is flying contains a varied population of water droplets such as a first population of droplets in the twenty micron size range (typical cloud droplets) and a second population of droplets in the fifty micron size range (SLD droplets). Further, while prior art ice detectors will advise the pilot that ice has formed on the detector, there is no way for the pilot to know whether ice is forming solely on the surfaces protected by the conventional ice protection systems or whether it is also forming on surfaces not serviced by these ice protection systems (except for visual inspections performed by the pilot, which are not always possible during flight.)
Because airline safety, in general, and passenger safety, in particular, are two of the foremost concerns of the airline industry, what is needed is an ice detector that can advise the pilot whether the aircraft is flying in an environment populated by water droplets typically found in clouds (namely, droplets having a mean diameter of about twenty microns) or whether the aircraft is flying in an environment populated by supercooled large droplets.